The use of compression type water chillers is the most common method of providing cooling for medium and large commercial and institutional buildings. Compression type water chillers are most commonly electric driven, but may also be driven by an engine or other power source. Electric driven water chillers are used extensively in individual buildings, campuses and district cooling plants to provide chilled water for comfort conditioning. Compression type water chillers have been employed for comfort conditioning for more than 75 years. There are several different types of compressors employed in water chillers, the centrifugal water chiller employs a centrifugal pump to compress the refrigerant and is generally the most efficient type for comfort conditioning purposes. Other types of water chillers include screw and scroll and reciprocating chillers which employ those types of compressors to compress the refrigerant.
FIG. 1 illustrates the major components of typical water chillers. In a compression type water chiller, a motor or engine (109), which is generally an electric motor, drives the compressor (110), which draws low pressure refrigerant gas from the cooler such as an evaporator (124), compresses it, and discharges it as a higher pressure hot gas via line (112) into a condenser (114). In the condenser, the hot gaseous refrigerant is condensed into a liquid by rejecting heat to tepid water from a cooling tower, or directly to outdoor air through a type of exchanger that is not shown. Water from the cooling tower (not shown) is received at condenser inlet (116) at, for example, 85 degrees F. The water leaves the condenser at outlet (118) at, for example, approximately 95 degrees F. having received heat rejected by the cooling refrigerant.
The condensed liquid refrigerant exits the condenser 114 at outlet 120 and flows through an expansion device (122) that regulates the flow into the cooler (124), which is held at a low pressure by the operation of the compressor (110). The expansion valve (122) is arranged to maintain a pressure differential between the condenser side and the cooler side of the valve. The low pressure environment in the cooler causes the refrigerant to change state to a gas and as it does so, it absorbs the required heat of vaporization from the chilled water circulating through the cooler. The low pressure vapor is drawn into the inlet of the compressor via line (130) and the cycle is continuously repeated. The chilled water is circulated through a distribution system by a pump (136) to water to air cooling coils (134) to cool air, or through radiant cooling panels, for comfort conditioning, or it is circulated through other devices or equipment to provide cooling for certain processes within the building. In general, we will refer to such cooling coils or similar devices as the load.
In the prior art, there are two common arrangements for connecting water chillers into chilled water supply and distribution systems. FIG. 2 shows a typical "single circuit" arrangement that was typically employed in earlier chilled water cooling systems. In this arrangement, a chilled water pump with check valve (210), (212), and (214) operates at a predetermined, constant flow rate, whenever its associated chiller (216), (218), and (220), respectively, is on, and prevents reverse flow when the pump is off. (Conversely, the pump is off when the corresponding chiller is off.) One or more condenser water pumps or direct outside air coils provide cooling for the condenser(s) in each chiller, but these are not shown as they are not significant to the present invention. These pumps and associated chillers together form a chilled water supply system (222).
The chilled water pump (210, 212, 214) provides water flow through the cooler of its associated chiller and to the cooling loads (230), (234), and (238) served by the cooling plant via a common supply line (224). The cooling loads, e.g. (230) are usually water to air coils that cool air serving a building, but they could be radiant cooling panels or process cooling loads. Each load is served by a corresponding three way valve (232), (236) and (240) respectively, that modulates to provide water flow through the corresponding load or bypass the flow directly back to the water chiller via a bypass line (233), (237) and (241), respectively. All of the return water flows via a common return line (244) back to the chilled water supply system (222). This arrangement provides variable flow through each load such that the cooling effect in each load can be modulated to meet the current demand, while at the same time assuring a constant flow through the cooler of the chiller(s) for stable operation.
Dual Circuit Arrangement
Another chiller arrangement that is widely employed in modern chilled water systems involves two water circuits; a constant flow primary chilled water circuit, and a variable flow secondary chilled water circuit. This arrangement is illustrated in FIG. 3. As in FIG. 2, the condenser circuit is not relevant to the invention and is not shown. The primary chilled water circuit operation is similar to the chilled water circuit in FIG. 2. A chilled water pump with check valve (310), (312), and (314) serves each chiller (316), (318) and (320) respectively. However, in FIG. 3, a separate secondary pump (328) and a decoupled secondary chilled water circuit is employed to provide chilled water flow to the loads served by the chiller plant. A decoupler line (326) ensures that differences in flows between the primary and secondary water circuits will not affect the operation of either circuit. Water flow in the decoupler line (326) will be in one direction (right to left in the figure) if the primary flow via supply line (324) exceeds the secondary flow (through the secondary pump (328), and in the opposite direction if secondary flow exceeds the primary flow. Thus the decoupler line serves as a bypass for both circuits, as needed to maintain constant flow in the primary circuit.
The primary/secondary pumping scheme of FIG. 3 has become the configuration of choice in recent years because it permits the use of variable flow two-way valves (332), (336), and (340) on the loads. In this configuration, as the requirement by the loads for cooling decreases, the water flow requirement through the secondary chilled water circuit is reduced, saving pumping power. A variable motor speed control (342) or some other pump flow modulating device is employed to adjust the flow as required in the secondary circuit. Like FIG. 2, this arrangement provides variable flow through each load such that the cooling effect in each load can be modulated to meet current demand while at the same time assuring a constant flow through the cooler of the chiller(s) in the primary circuit (322) for stable operation.
In all chiller configurations there is an emphasis on maintaining a constant flow of chilled and condenser water through the chiller at all times. For many years it was considered essential to maintain constant water flow through the chiller coolers and condensers. There are several reasons why constant water flow through the evaporators and condensers was thought to be important. One important reason is that it was thought that chiller efficiency would be adversely affected if chilled water flow were reduced under any circumstances. Recent tests, however, have shown that chiller efficiency is not necessarily adversely affected by reducing chilled water flow. It has been shown that the efficiency of at least one type of chiller is virtually identical as chilled water flow is varied from as low as 2.5 fps (feet per second) to well over 9 fps. The actual range may be even higher.
Now that it is known that chilled water flow can be varied in chillers without loss of efficiency, new simpler chiller configurations that require less pumping power at reduced loads have been suggested. Such a configuration is shown in FIG. 4. In FIG. 4, multiple chillers (411), (412), and (413) can be operated with a single chilled water pump (417) that is connected to a variable speed motor drive (418) with two way valves (431), (432), and (433) employed to modulate water flow through the loads (434), (435), and (436). In Figure four, only a single pump is required, and the water circuit is very similar to that of FIG. 1 except the system adjusts flow through the chillers as well as the loads. As the load decreases, the need for chilled water flow is decreased and the pump is slowed down, reducing energy use. As flow decreases, individual chillers can be shut down and flow through them is stopped by closing their associated valve (414), (415), and (416). Flow rate thresholds are established for starting or stopping additional chillers and when the flow through chillers sequenced on is less than the stop threshold for the total chillers on, one of the "on" chillers is sequenced off. This is a simpler and more energy efficient configuration than in use today, but it is only rarely employed because the variable flow of water through the chillers has the potential of unstable control of the chiller in response to load changes.
Controlling Chiller Plant Operation
The standard method of control of nearly all water chillers, no matter what type of configuration they are in, is to operate them to maintain a specific chilled water temperature leaving the chiller. A control diagram of this type is shown in FIG. 5. In some installations the chilled water setpoint is reset according to conditions at the load to make the chiller operate more efficiently, particularly during periods of low load. It is known that elements of a chiller plant can be considered together for operation to optimize the overall efficiency of the system. Examples of such prior art are U.S. Pat. No. 5,600,960, in which a cooling tower leaving water temperature is calculated and maintained, and U.S. Pat. No. 4,327,559, in which the chilled water temperature is adjusted to optimize the overall energy use of a chiller and air system.
It is also known to use a microprocessor or other type controller to control the capacity of the compressor in a chiller in response to supply chilled water temperatures as shown in the chiller control diagram of FIG. 5. In FIG. 5, a water temperature sensor (510) located near the chilled water outlet (511) senses the chilled water temperature leaving the chiller. A similar water temperature sensor (512) located near the chilled water inlet (513) senses the chilled water temperature returning to the chiller from the loads. A controller (514) regulates the operation of the compressor (515), and in some cases the speed of the compressor motor or engine (516), in various ways depending on the type of compressor employed, to increase or decrease the cooling effect being produced by the chiller so as to maintain a constant preset chilled water temperature setpoint.
The inlet chilled water temperature sensor (512) is employed to stabilize the control by sensing changes in the load served by the chiller. If the return chilled water temperature rises, the controller increases the chiller capacity because the load is increasing, and conversely, if the return chilled water temperature falls, the controller decreases the chiller capacity because the load is decreasing. One example of such a prior art feedback system is shown in U.S. Pat. No. 4,274,264. Other methods are employed as well to stabilize chiller control. Stabilizing control based on chilled water temperature rate of change (U.S. Pat. No. 3,780,532), and by providing a change in capacity based on deviation from setpoint of the chilled water temperature (U.S. Pat. No. 4,589,060) are known.
However, if the chilled water flow through the chiller cooler becomes variable, then the ability of any of these methods to accurately predict or adjust to changes in the load are lost and unstable operation may develop. For example, in a variable flow distribution system, an increase in return chilled water temperature may be the result of a decrease in flow and not an increase in load. In fact, the load may actually be decreasing, which is causing the flow to decrease. The resulting instability in chiller control in such circumstances has resulted in the avoidance of the simpler and more efficient chiller configurations, and is effectively blocking the implementation of more efficient variable flow chiller plant configurations.